1. Field of the Invention
The present invention is related to a light-emitting diode lighting device, and more particularly, to a light-emitting diode lighting device with overvoltage protection.
2. Description of the Prior Art
Compared to traditional incandescent bulbs, light-emitting diodes (LEDs) are advantageous in low power consumption, long lifetime, small size, no warm-up time, fast reaction speed, and the ability to be manufactured as small or array devices. In addition to outdoor displays, traffic signs, and LCD backlight for various electronic devices such as mobile phones, notebook computers or personal digital assistants (PDAs), LEDs are also widely used as indoor/outdoor lighting devices in place of fluorescent of incandescent lamps.
FIG. 1 is a diagram illustrating the voltage-current chart of a light-emitting diode. When the forward-bias voltage of the light-emitting diode is smaller than its barrier voltage Vb, the light-emitting diode functions as an open-circuited device since it only conducts a negligible amount of current. When the forward-bias voltage of the light-emitting diode exceeds its barrier voltage Vb, the light-emitting diode functions as a short-circuited device since its current increases exponentially with the forward-bias voltage. The barrier voltage Vb, whose value is related to the material and doping type of the light-emitting diode, is typically between 1.5 and 3 volts. For most current values, the luminescence of the light-emitting diode is proportional to the current. Therefore, a current source is generally used for driving light-emitting diodes in order to provide uniform luminescence.
FIG. 2 is a diagram of a prior art LED lighting device 950. The LED lighting device 950 includes a power supply circuit 110, a resistor R and a luminescent device 10. The power supply circuit 110 is configured to receive an alternative-current (AC) voltage VS having positive and negative periods and convert the output of the AC voltage VS in the negative period using a bridge rectifier 112, thereby providing a rectified AC voltage VAC, whose value varies periodically with time, for driving the luminescent device 10. The resistor R is coupled in series with the luminescent device 10 for regulating its current ILED. In many applications, multiple light-emitting diodes are required in order to provide sufficient brightness. Since a light-emitting diode is a current-driven device whose luminescence is proportional to its driving current, the luminescent device 10 normally adopts a plurality of light-emitting diodes D1-Dn coupled in series. Assuming that the barrier voltage of all the light-emitting diodes D1-Dn is equal to the ideal value Vb and the rectified AC voltage VAC varies between 0 and VMAX with time, a forward-bias voltage larger than n*Vb is required for turning on the luminescent device 10. Therefore, the energy between 0 and n*Vb cannot be used. As the number of the light-emitting diodes D1-Dn increases, a higher forward-bias voltage is required for turning on the luminescent device 10, thereby reducing the effective operational voltage range of the LED lighting device 950; as the number of the light-emitting diodes D1-Dn decreases, the large driving current when VAC=VMAX may impact the reliability of the light-emitting diodes. Therefore, the prior art LED lighting device 950 needs to make compromise between the effective operational voltage range and the reliability. Meanwhile, the current-limiting resistor R also consumes extra power and may thus lower system efficiency.
FIG. 3 is a diagram of another prior art LED lighting device 960. The LED lighting device 960 includes a power supply circuit 110, an inductor L, a capacitor C, a switch SW, and a luminescent device 10. The power supply circuit 110 is configured to receive an AC voltage VS having positive and negative periods and convert the output of the AC voltage VS in the negative period using a bridge rectifier 112, thereby providing a rectified AC voltage VAC, whose value varies periodically with time, for driving the luminescent device 10. The inductor L and the switch SW are coupled in series with the luminescent device 10 for limiting its current ILED. The capacitor C is coupled in parallel to the luminescent device 10 for absorbing voltage ripples of the power supply circuit 110. For the same current-regulating function, the inductor L consumes less energy than the resistor R of the LED lighting device 950. However, the inductor L for regulating current and the capacitor for stabilizing voltage largely reduce the power factor of the LED lighting device 960 and the energy utilization ratio. Therefore, the prior art LED lighting device 960 needs to make compromise between the effective operational voltage range and the brightness. Also, the inductor L, the capacitor C and the switch SW may occupy large space and require separate assembly steps.